Daniel Sperling is director of the Institute of Transportation Studies; associate director of the Energy Efficiency Center and professor of Transportation Engineering and Environmental Policy at the University of California, Davis.

Do you think the government is doing enough to find alternative energy solutions for gasoline cars?

No. The government needs to send signals that it is serious about reducing oil use. The best way to do that is to do one or more of the following: place higher taxes on petroleum fuels, charge higher fees or taxes on gas guzzlers, require or strongly encourage car companies to improve the fuel economy of vehicles, and provide incentives to car and energy companies to develop and market alternative fuels and alternative fuel vehicles.

How about the automobile companies?

Two car companies, Toyota and Honda, are investing aggressively in hybrid electric technology and a few others are investing more modestly. Some companies are pursuing diesel engine technology, which is more energy efficient than gasoline. And some are investing in fuel cell technology. What is needed are incentives for them to increase those investments.

President Bush has voiced his commitment to hydrogen-powered fuel cell cars, including a $1.7 billion hydrogen research program first detailed in 2003. How do these cars work?

Future Cell Vehicles are electric cars. Hydrogen is pumped into a tank in the car, just as with gasoline. The hydrogen gas is then fed into the fuel cell where it is electrochemically converted into electricity -- with no combustion, no moving parts, and no emissions other than water vapor. The electricity is used to power the vehicle.

How viable are they as an alternative to gasoline-powered cars?

In many ways, they are more viable than gasoline. A fuel cell electric vehicle is better suited to modern vehicles that increasingly use electrical systems in place of mechanical and hydraulic to steer, brake, and control the various functions of the vehicle. Also, in a fuel cell vehicle, the entire powertrain can be consolidated into a flat "skateboard" chassis, providing automakers much design freedom in latching all sorts of different vehicle bodies on to the chassis -- without having to work around a protruding, heat-producing engine and large mechanical driveline. A fuel cell is also 2-3 times more energy efficient than a gasoline engine.

When can we expect to see them in the future?

There is at least one automaker planning to begin selling them to the public at a reasonable price as early as 2009, and several aiming for a few years after that. But this depends on fuel suppliers building more hydrogen fuel stations and the government offering incentives to buyers (as they do now for hybrid vehicles).

Sales of hybrids are increasing; do you think the trend will continue?

Absolutely. The real question is how much and how fast. And that depends on how fast automakers can bring down the costs and how many customers are willing to pay how much extra for an energy-efficient vehicle.

What other fuels might we use in our automobiles in the future?

The two other major possibilities are electricity (charged from a grid and stored on the vehicle) and ethanol (or methanol). Electricity may be used in vehicles with large batteries, or in so-called plug-in hybrids where a small combustion engine is added to provide longer driving range between charges. I believe we will see many of our vehicles running solely on electricity in the not-so-far-off future. They will tend to be smaller cars used for short trips in neighborhoods and various types of campuses (for example, military and universities). Note that most households own 2 or more cars, so devoting one of them to short, local trips is not so outlandish. Plug-in hybrids combine batteries and combustion engines to provide performance and range similar to gasoline cars.

The other major option is liquid fuels made from biological material. Today, ethanol is made from corn. A better option is to convert materials such as grasses, trees, and crop wastes (such as rice straw) to ethanol (or methanol or a petroleum-like liquid). The process is more complex and expensive than converting corn into ethanol, but it requires far less land and energy, and produces far fewer greenhouse gases.